air supply with system demand (although not always in real time) and are one of the most important determinants of overall system energy efficiency. This fact sheet discusses both individual compressor control and overall system control of plants with multiple compressors. Proper control is essential to efficient system operation and high performance. The objective of any control strategy is also to shut off unneeded compressors or delay bringing on additional compressors until needed. All units that are operating should be run at full-load, except one unit for trimming.
Compressor systems are typically comprised of multiple compressors delivering air to a common plant air header. The combined capacity of these machines is generally sized to meet the maximum plant air demand. System controls are almost always needed to orchestrate a reduction in the output of the individual compressor(s) during times of lower demand. Compressed air systems are usually designed to operate within a fixed pressure range and to deliver a volume of air that varies with system demand. System pressure is monitored and the control system decreases compressor output when the pressure reaches a predetermined level. Compressor output is then increased again when the pressure drops to a lower predetermined level.
The difference between these two pressure levels is called the control range. Depending on air system demand, the control range can be anywhere from 2 to 20 psi. In the past, individual compressor controls and non-supervised multiple machine systems were slow and imprecise. This resulted in wide control ranges and large pressure swings. As a result of these large swings, individual compressor pressure control set points were established to maintain pressures higher than needed. This ensured that swings would not go below the minimum requirements for the system. Today, faster and more accurate microprocessor-based system controls and variable speed compressors with tighter control ranges allow for a drop in the system pressure set points. Precise control systems are able to maintain lower average pressure without going below minimum system requirements.
A rule of thumb for systems in the 100 psig range is: for every 2 psi increase in discharge pressure, energy consumption will increase by approximately 1 percent at full output flow (check performance curves for centrifugal and two-stage, lubricant-injected, rotary screw compressors). There is also another penalty for higher-than-needed pressure. Raising the compressor discharge pressure increases the demand of every unregulated usage, including leaks, open blowing, etc. Although it varies by plant, unregulated usage is commonly as high as 30 to 50 percent of air demand. For systems in the 100 psig range with 30 to 50 percent unregulated usage, a 2 psi increase in header pressure will increase energy consumption by about another 0.6 to 1.0 percent because of the additional unregulated air being consumed. The combined effect results in a total increase in energy consumption of about 1.6 to 2 percent for every 2 psi increase in discharge pressure for a system in the 100 psig range with 30 to 50 percent unregulated usage.
Caution must be taken when lowering average system header pressure, because large, sudden changes in demand can cause the pressure to drop below minimum requirements, which can lead to improper functioning of equipment. With careful matching of system controls and storage capacity, these problems can be avoided.
Controls and System Performance Controls and System Performance
Few air systems operate at full-load all of the time. Part-load performance is therefore critical, and is primarily influenced by compressor type and control strategy. The type of control specified for a given system is largely determined by the type of compressor being used and the facility's demand profile. If a system has a single compressor with a very steady demand, a simple control system may be appropriate. On the other hand, a complex system with multiple compressors, varying demand, and many types of end uses will require a more sophisticated strategy. In any case, careful consideration should be given to both compressor and system control selection because they can be the most important factors affecting system performance and efficiency.
Compressed Air System Controls
Compressed Air System Controls
5–Compressed Air System Controls 5–Compressed Air System Controls
Individual Compressor Control Strategies Individual Compressor Control Strategies
Over the years, compressor manufacturers have developed a number of different types of control strategies. Controls, such as start/stop and load/unload, respond to reductions in air demand, increasing compressor discharge pressure by turning the compressor off or unloading it so that it does not deliver air for periods of time. Modulating inlet and multi-step controls allow the compressor to operate at part-load and deliver a reduced amount of air during periods of reduced demand.
Start/Stop
Start/Stop.Start/stop is the simplest control available and can be applied to either reciprocating or rotary screw compressors. The motor driving the compressor is turned on or off in response to the discharge pressure of the machine. Typically, a simple pressure switch provides the motor start/stop signal. This type of control should not be used in an application that has frequent cycling, because repeated starts will cause the motor to overheat and other compressor components to require more frequent maintenance. This control scheme is typically only used for
applications with very low-duty cycles for compressors in the 30 horsepower (hp) and under range. Its advantage is that power is used only while the compressor is running, but this is offset by having to compress to a higher receiver pressure to allow air to be drawn from the receiver while the compressor is stopped.
Load/Unload
Load/Unload.Load/unload control, also known as constant-speed control, allows the motor to run continuously, but unloads the compressor when the discharge pressure is adequate. Compressor manufacturers use different strategies for unloading a compressor, but in most cases, an unloaded rotary screw compressor will consume 15 to 35 percent of full-load horsepower while delivering no useful work. As a result, some load/unload control schemes can be inefficient.
Modulating Controls
Modulating Controls.Modulating (throttling) inlet control allows the output of a compressor to be varied to meet flow requirements. Throttling is usually accomplished by closing the inlet valve, thereby restricting inlet air to the compressor. This control scheme is applied to centrifugal and lubricant-injected rotary screw compressors. This control method cannot be used on reciprocating or lubricant-free rotary screw compressor and when applied to lubricant-injected rotary screw compressors, is an inefficient means of varying compressor output. When used on centrifugal compressors, more efficient results are obtained,
particularly with the use of inlet guide vanes, which direct the air in the same direction as the impeller inlet. However, the amount of capacity reduction is limited by the potential for surge and minimum throttling capacity.
Inlet-valve modulation used on lubricant-injected rotary air compressors allows compressor capacity to be adjusted to match demand. A regulating valve senses system or discharge pressure over a prescribed range (usually about 10 psi) and sends a proportional pressure to operate the inlet valve. Closing (or throttling) the inlet valve causes a pressure drop across it, reducing the inlet pressure at the compressor and, hence, the mass flow of air. Since the pressure at the compressor inlet is reduced while discharge pressure is rising slightly, the compression ratios are increased so that energy savings are somewhat limited.
Inlet valve modulation is normally limited to a range of from 100 percent to about 40 percent of rated capacity. When operating at 40 percent rated capacity and when discharge pressure reaches full load pressure plus 10 psi, it is assumed demand is insufficient to require continued air discharge to the system. At this point the compressor will operate fully unloaded, like a compressor using load/unload controls.
Dual-Control/Auto-Dual
Dual-Control/Auto-Dual.For small reciprocating compressors, dual-control allows the selection of either start/stop or load/unload. For lubricant-injected rotary screw compressors, auto-dual control provides modulation to a preset reduced capacity followed by unloading with the addition of an overrun timer to stop the compressor after running unloaded for a pre-set time.
Variable Displacement
Variable Displacement.Some compressors are designed to operate in two or more partially loaded conditions. With such a control scheme, output pressure can be closely controlled without requiring the compressor to start/stop or load/unload.
Reciprocating compressors are designed as two-step (start/stop or load/unload), three-step (0, 50, 100 percent) or five-step (0, 25, 50, 75, 100 percent) control. These control schemes generally exhibit an almost direct relationship between motor power consumption and loaded capacity.
Some lubricant-injected rotary screw compressors can vary their compression volumes (ratio) using sliding or turn valves. These are generally applied in conjunction with modulating inlet valves to provide more accurate pressure control with improved part-load efficiency.
5–Compressed Air System Controls 5–Compressed Air System Controls
Variable Speed Drives
Variable Speed Drives.Variable speed is accepted as an efficient means of rotary compressor capacity control, using integrated variable frequency AC or switched reluctance DC drives. Compressor discharge pressure can be held to within +/- 1 psi over a wide range of capacity, allowing additional system energy savings.
Rotary screw compressors with fixed-speed drives can only be stopped and started a certain number of times within a given time frame. Depending on the control scheme used, instead of stopping the compressor, it will be unloaded, throttled, or the compressor displacement will be varied in applications where the demand for air changes over time. In some cases, these control methodologies can be an inefficient way to vary compressor output. Compressors equipped with variable speed drive controls continuously adjust the drive motor speed to match variable demand requirements.
In a positive-displacement rotary compressor, the displacement is directly proportional to the rotational speed of the input shaft of the air end. However, it is important to note that with constant discharge pressure, if efficiency remained constant over the speed range, the input torque requirement would remain constant, unlike the requirement of dynamic compressors, fans, or pumps. The actual efficiency also may fall at lower speeds, requiring an increase in torque. Electric motors and controllers are currently available to satisfy these needs, but their efficiency and power factor at reduced speeds must be taken into consideration. Steam turbines and engines also are variable speed drivers but are rarely used to power industrial air compressors.
Capacity Controls for Centrifugal Type Compressors. Capacity Controls for Centrifugal Type Compressors.
Centrifugal compressors have complex characteristics affected by inlet air density and interstage cooling water temperature. The basic characteristic curve of head (pressure) versus flow is determined by the design of the impeller(s). Radial blades produce a very low rise in head as flow decreases. Backward leaning blades produce a steeper curve. The greater the degree of backward leaning, the steeper the curve.
Two potential occurrences must be avoided. The first is surge, caused by a flow reversal, which can occur at low flow rates and can cause damage to the compressor. Surge is avoided by limiting the amount of flow reduction. The second is choke at flow rates above design, when the velocity of the air at the
impeller inlet approaches the speed of sound and flow and head cannot be sustained. Choke, or stonewall, is normally avoided by sensing drive motor amps and using this signal to limit the flow rate.
The flow rate can be reduced by an inlet throttle valve, which reduces the pressure at the inlet to the first stage impeller. This reduces the mass flow in direct proportion to the absolute inlet pressure. The inlet air density also is reduced, resulting in reduced pressure head produced by the impeller.
A variation of this uses inlet guide vanes, which also reduce the mass flow and the inlet air density, but cause the air to be directed radially towards the impeller inlet in the same direction as the impeller rotation. This improves the efficiency compared with simple throttling. In some cases, discharge bypass or blow-off control is used to limit flow delivered to the compressed air system. Compressed air is discharged to the atmosphere through a discharge silencer or cooled and returned to the compressor inlet. This is a waste of energy and should be used only where necessary.
Multiple Compressor Control Multiple Compressor Control
Systems with multiple compressors use more sophisticated controls to orchestrate compressor operation and air delivery to the system. Network controls use the on-board compressor controls’ microprocessors linked together to form a chain of communication that makes decisions to stop/start, load/unload, modulate, vary displacement, and vary speed. Usually, one compressor assumes the lead with the others being subordinate to the commands from this compressor. System master controls coordinate all of the functions necessary to optimize compressed air as a utility. System master controls have many functional capabilities, including the ability to monitor and control all components in the system, as well as trending data to enhance maintenance functions and minimize costs of operation. Other system controllers, such as pressure/flow controllers, can also substantially improve the performance of some systems.
The objective of an effective automatic system control strategy is to match system demand with compressors operated at or near their maximum efficiency levels. This can be accomplished in a number of ways, depending on fluctuations in demand, available storage, and the characteristics of the equipment supplying and treating the compressed air.
5–Compressed Air System Controls 5–Compressed Air System Controls
Network Controls.
Network Controls.Less sophisticated network controls use the cascade set point scheme to operate the system as a whole. Those systems are capable of avoiding part load compressors but can still present the problem of approaching production’s minimum pressure requirement as more and more compressors are added and the range of compressor load and unload set points increases.
The more sophisticated network control systems use single set-point logic to make their operational decisions to start/stop, etc. In systems with positive- displacement compressors (reciprocating, rotary screws, etc.) all compressors are kept fully loaded except for one compressor that is operated in some part-load fashion specific to the design of the machine.
Three major disadvantages of network system controls are:
• They are capable of controlling only air compres sors. • They cannot be networked with remote compressor
rooms without a master control of some type. • Typically they only work with comp ressors of the
same brand and configuration because of micro processor compatibility issues.
Expensive upgrades or retrofits may be required to make different brands of compressors or older versions of the same brand work in the system. In some cases, retrofits are not available and a different brand or outdated compressors cannot be used in the control scheme.
There are no network controls available that can coordinate the control of rotary screw, reciprocating, and centrifugal compressors as one system. To do this, system master controls are required, particularly if there is a desire to monitor and operate compressors, cooling systems, dryers, filters, traps, storage, pressure/ flow controllers, and any other part of a compressed air system that a facility might want included in the control scheme.
System Master Controls.
System Master Controls.If complexity outpaces the capabilities of local and network controls, a system master control is required to coordinate all of the functions necessary to optimize compressed air as a utility. System master controls have many functional capabilities, including the ability to monitor and control all components in the system, as well as to trend data to enhance maintenance functions and minimize costs of operation. System master controls interface
with all brands and types of air compressors, and can coordinate the operation of satellite compressor rooms spread around the facility, or in different buildings across an industrial campus. The primary function of these controls, as with the network controls, is to operate a multiple compressor system in harmony. The least sophisticated have few if any of the features mentioned above and use cascading set-point logic to control compressors. The most sophisticated, state-of- the-art system master controls use single-point control logic with rate-of-change dynamic analysis to make decisions regarding how the compressed air system responds to changes. These changes can occur on the demand side, supply side, or in the ambient conditions. All affect the performance of the system and have a role in how the system should respond. Some of these require short duration support, such as additional storage. Others require additional compressor power to be brought online, and some will require a combination of both.
A properly configured system master control can determine the best and most energy-efficient response to events that occur in a system. The number of things a system master control can interface with is governed by practicality and cost limitations. System master control layout has the capability to perform these functions:
• Send/receive communications
• Communicate with a plant information system • Monitor weather conditions to adjust operational
parameters
• Adjust pressure/flow controller set points • Monitor dryer dew point(s)
• Monitor filter differential pressure • Monitor condensate trap function • Start/stop and load/unload compressors • Change base/trim duties
• Select appropriate mix of compressors to optimize efficiency
• Select which compressor should be started/stopped relative to change in system demand.
Typically it will cost $300 to $1,500 in hardware per data collection point. Some of the latest system master controls integrate the functions of real-time pricing systems, peak shaving with natural gas driven compressors, and aggregate system operation.
5–Compressed Air System Controls 5–Compressed Air System Controls
Pressure/Flow Controllers Pressure/Flow Controllers
Pressure/flow controllers are system pressure controls used in conjunction with the individual compressor or system controls described previously. A pressure/flow controller does not directly control a compressor and is generally not included as part of a compressor package. This is a device that serves to separate the supply side of a compressor system from the demand side. This may require compressors to be operated at an elevated pressure and therefore, increased horsepower (but possibly fewer compressors and/or shorter periods of operation), while pressure on the demand side can be reduced to a stable level to minimize actual compressed air consumption.
Storage, sized to meet anticipated fluctuations in demand, is an essential part of the control strategy. Higher pressure supply air enters the primary storage tanks from the air compressors and is available to reliably meet fluctuations in demand at a constant lower pressure level.
To function properly, pressure/flow controllers require a properly sized primary air receiver located upstream. The pressure swing caused by the multiple compressor control band is relegated to the primary receiver, while the pressure/flow controller controls the plant header pressure in a lower, much narrower